Lag oscillations of a hinged blade of a rotorcraft rotor are generally damped little and they take place at a frequency that is low; this results in a risk of coupling with other oscillations or vibrations that occur at similar frequencies, in particular with resonant modes in deformation of the rotorcraft structure; such coupling runs the risk of damaging or destroying components of the rotorcraft; such oscillations lead to high levels of useless forces or stresses.
Consequently, the rotors of a rotorcraft are generally fitted with means or damping such oscillations, which means are generally referred to as lag dampers or as frequency adapters.
The purpose of such damper means is to reduce the amplitude of lag oscillations by dissipating a portion of the energy of such oscillations; when the damper means present non-negligible stiffness, this stiffness changes a response frequency of the assembly constituted by the hinged blade and its lag damper, which is why it can be referred to as a “frequency adapter”.
The terms “damper” and “adapter” are used interchangeably below to designate a device or a system that serves to reduce or to adapt the amplitude of the lag oscillations of a hinged blade.
Such dampers generally include a structure of a polymer material that presents high capacity for elastic deformation and that is generally referred to as “elastomer” or “synthetic rubber”.
Such dampers are described in particular in the following patents: FR 2 063 969, FR 2 111 845, FR 2 127 061, FR 2 672 947, FR 2 677 723, and WO 94/15113.
In general, a rotorcraft rotor has as many lag dampers as it has blades; when a lag damper connects a blade to the rotor hub, the connections between the damper and the blade and between the damper and the hub are made in respective portions of the blade and of the hub that are reinforced for this purpose; as a result, the weight, the complexity, and the cost of a rotor are increased.
When the damper interconnects two adjacent blades of the rotor, such a damper being referred to as an “inter-blade” damper, each blade needs to have two reinforced portions enabling it to be fastened respectively to two adapters.
Furthermore, under such circumstances, the lag dampers cannot damp lag oscillations of the blades that correspond to a “collective” lag mode in which the oscillations of the blades are synchronous and in-phase, since relative displacements between the blades are then zero.
Since collective lag mode is then ignored and not damped, there is an increased risk of coupling with the power unit (rotor—main gearbox—engines), so it is necessary to install lag stops on the hub in order to limit the amplitude of blade movement whenever starting or stopping the rotor.
U.S. Pat. No. 5,372,478 describes a rotor having a single damper assembly contained in a casing at the center of a hollow hub of the rotor. The assembly includes a series of elastomer layers alternating with plates; the elastomer is stressed in shear; each plate is connected via a short rod to the inner end of the corresponding blade; the casing (together with the damper assembly) is either secured rigidly to the hub, or it is pivotally mounted relative thereto, or else it is floatingly mounted, being suspended from the ends of the blades.
Such a damper system presents the drawback of being sensitive to flap oscillations of the blades.